Methods of making artificial tooth onlays and inlays

ABSTRACT

The present invention relates to artificial onlay tooth crowns or inlays composed of a prefabricated core designed for preparations for onlay tooth crowns or inlays in natural teeth. The core is preferably fabricated from a high strength densely sintered ceramic material by copy milling from a negative reproduction from the prepared cavity to a compacted body or a presintered ceramic material. During the copy milling, the sintering shrinkage is considered by enlargement of the copy milled compacted body or the presintered body corresponding to the sintering shrinkage. The onlay tooth crown and inlays are given the final shape by shaping the surfaces outside the cavity of the compacted or the presintered body. After the final sintering, the external surface can be shaped and a veneer material attached to the external surface of the core by, e.g., firing of dental porcelain. The manufacture of onlay tooth crowns or an inlay according to the method of the invention decreases essentially the manufacturing time for onlay tooth crowns and inlays and at the same time the strength and the accuracy to shape increase.

BACKGROUND OF THE INVENTION

This invention relates to accurately shaped artificial, all ceramiconlay tooth crowns and inlays as replacement for lost tooth structure.By starting from a negative reproduction of the prepared tooth and thencopy milling a surface which will fit into the prepared cavity, a toothcrown or an inlay is obtained which is easier to produce and of lowercost. In addition, the onlay crown or inlay has higher strength and amore accurate shape. Preferably the coping is manufactured from abiocompatible, high strength ceramic material, which is sintered to highdensity.

Artificial tooth crowns and inlays made from a metal are todaymanufactured mainly in the following way: A dentist makes a preparationon a tooth, on which a dental construction is to be fixed in the mouthof a patient, an impression is made and with this impression a copy ofthe preparation is made in gypsum. On this model, a dental technicianprepares a crown in wax. The adjacent teeth must be considered, and thedental technician must have models from the two jaws. A sprue former ofwax is fixed on one of the cusps of the wax crown. The wax crown isloosened from the gypsum model. The wax crown with the sprue former areinvested in a metal ring with investment. The wax is burnt out and acrown can be cast in a precious or non-precious metal. The cast crowncan, in certain cases, be covered by a veneer made of porcelain in orderto obtain a color of the tooth crown similar to the color of naturalteeth. Instead of porcelain, plastic material can be used.

The fabrication of tooth crowns in glass is very close to the techniquedescribed above with the difference that after the casting, a thin layerof porcelain is painted on the surface and fired in order to give thetooth crown individual tooth colors.

Tooth crowns fabricated mainly of porcelain can be made withconventional dental porcelain technique from a sheet made of a preciousalloy. Porcelain crowns and inlays can also be made with conventionaldental porcelain technique on a model of the abutment. The material ofthis model does not change dimensions on heating up to 1200° C. When thetooth crown or the inlay is ready, the model of the abutment is removedby sand blasting.

The above described complicated and time consuming methods are used tomanufacture crowns and inlays, which will fit in individually preparedcavities in natural teeth.

The problem with the material now used (porcelain, glass, etc.) inartificial tooth crowns is their brittleness, which often gives earlyfracture, and these artificial crowns and inlays must be replaced moreor less regularly.

OBJECTS AND SUMMARY OF THE INVENTION

One object of the present invention is to provide artificial toothcrowns and inlays, which are easier and accordingly cheaper to make and,in addition, have higher strength and more accurate shape and a methodof making such tooth crowns and inlays.

Another object is to make an inlay or an onlay tooth crown by usingdensely sintered, high strength ceramic material, provided that thedemand of high strength, accuracy to shape (which includes compensationfor any shrinkage during sintering) can be combined with the demands onthe application of porcelain concerning burning, adherence,biocompatibility and esthetics.

In accordance with one aspect of the present invention there is providedan artificial onlay tooth crown or inlay for fit into a prepared toothcavity comprising a core and a veneer overlying at least part of thecore which does not fit into the said cavity, the boundary surface ofthe core which fits into said cavity being formed by copy milling anegative reproduction of the said cavity.

In accordance with another aspect of the present invention there isprovide a method of making an artificial onlay tooth crown or inlay forfit into a prepared tooth cavity comprising: forming a negativereproduction of said cavity; copy milling a core from said negativereproduction including a surface abutting said cavity and an externalsurface; and applying a veneer to at least part of the said externalsurface of said core.

In accordance with a further aspect of the present invention there isprovided a method of filling a tooth cavity with an artificial onlaytooth crown or inlay which comprises forming an artificial onlay toothcrown or inlay by the method of the preceding paragraph and adhesivelyfitting said onlay tooth crown or inlay into said tooth cavity.

BRIEF DESCRIPTION OF THE DRAWINGS

FIG. 1 is a schematic representation of a natural tooth with anartificial inlay (dotted).

FIG. 2 is a representation of a cross section of a natural tooth with aninlay.

FIG. 3 is a representation of a cross section of a root-filled naturaltooth with an inlay.

FIG. 4 is a schematic representation of a natural tooth with an onlaytooth crown (dotted).

FIG. 5 is a representation of a cross section of a natural tooth with anonlay tooth crown.

FIG. 6 is a representation of a cross section of a root-filled toothwith an onlay tooth crown.

DETAILED DESCRIPTION OF THE PREFERRED EMBODIMENTS

FIG. 1 shows an artificial inlay (shown as the dotted portion) in anatural tooth. In the Figure, the border between the cavity E and theinlay core B is indicated as I, the axis of the tooth as X.

FIG. 2 shows a cross section of an inlay in a natural tooth. In thisFigure, the veneer is indicated as A, the core made from dense sinteredceramic as B, the cement as C, the copy milled surface of the core as D,the prepared surface of cavity as E, the pulp as F, and the naturaltooth root as H.

FIG. 3 shows a cross section of an alternative design for a root-filledtooth, the same letters meaning the same things as in FIG. 2, while aroot filling is indicated as G.

FIG. 4 shows an artificial onlay in a natural tooth, the onlay being thedotted portion while FIG. 5 shows a cross section of an onlay toothcrown on a natural tooth, and FIG. 6 shows a cross section of analternative design for an onlay tooth crown for a root-filled tooth, thesame letters meaning the same things as with FIGS. 1, 2 and 3.

In each of the Figures, the artificial onlay tooth crown or the inlayare the combination of the core B with the veneer A. They are fixed inthe prepared cavity E with cement C.

According to the present invention, artificial onlay tooth crowns andinlays are made in which the core is prefabricated from a biocompatiblematerial with the part D of the outer surface of the core given suchdimensions as to fit into a prepared cavity E, which can be apreparation for an inlay or a preparation for an onlay tooth crown or apreparation for a tooth crown on a root canal pin. A biocompatiblematerial for the core is a material which is not toxic and does notcause damage to oral tissues or does not give unwanted system effects.In addition this material must not discolor or otherwise give unwantedeffects to the veneer material. The onlay tooth crowns and the inlaysare fixed by cementation in the cavities.

The present invention considerably simplifies the handicraft of thedental technician. With the aid of gypsum models of the two jaws andwith the prefabricated core placed on the model of the toothpreparation, a dental technician can make the final design of the onlaytooth crown or inlay and at the same time control their function andsize. In order to make the veneer, a porcelain furnace or an apparatusfor pressing of composite veneer is needed. The method of making a toothcrown or an inlay to fit into an existing preparation according to thepresent invention essentially decreases the production time for theseconstructions and at the same time increases its strength and accuracyof shape.

Cores according to the invention can be made from metals or alloys(e.g., titanium or dental alloys), plastic, porcelain, glass-ceramic orapatite-ceramic by copy milling a negative reproduction of the preparedcavities (E). Copy milling accurately matches the boundary surface D ofthe onlay tooth crown or of the inlay to the cavity E. A negativereproduction is made by making an impression of the prepared tooth or amodel from this tooth together with its adjacent teeth. The impressionmaterial fills up the whole cavity and when the impression is removedfrom the teeth, this impression comprises the surface D and the contactpoints to the adjacent teeth. From the impression, material closest tosurface D is adjusted in order to have this surface D within reach forcopy milling. The contacts with the adjacent teeth give the limitationin mesial distal direction. The contour outside the cavity is preparedmanually with conventional dental technical machining practice. Anegative reproduction can be, e.g., a Kerr impression or a siliconimpression.

Preferably, the core according to the invention is made from abiocompatible densely sintered, high strength ceramic material. On theobtained core, the veneer is built up, so that the final product will bea tooth crown or an inlay fitting into an existing preparation and tothe actual whole set of teeth.

As can be seen from FIG. 2 and FIG. 3, artificial inlays are made as acore of densely sintered ceramic B with veneer A. They are fixed in theprepared cavity E by, e.g., cementing. The thin layer of cement Cconnects the prepared cavity walls E with that part of the surface D ofthe inlay or the onlay tooth crown, which has been made so that thissurface D fits with great precision into the prepared cavity E. Thelayer of cement can have a thickness <200 μm, preferably 25-75 μm.Artificial onlay tooth crowns are made as a core of densely sinteredceramic B with veneer A. As can be seen from FIG. 6, the preparation ofa root filled tooth can be extended down into the root canals in orderto have optimum retention of the tooth crown. The veneer A can be madefrom dental porcelain or plastic. The cementing of the constructions canbe made with conventional adhesives with, e.g., glassionomercement,phosphatecement, or a resin. In the latter case, it can be an advantageto silane treat the surfaces D of the constructions which will be joinedwith the prepared surfaces of the tooth structure. The enamel walls ofthe prepared cavity can be etched and the dentin walls of the preparedcavity E can be treated with dentine adhesive before the above describedrestorations are cemented with resin, which resin can contain fillerparticles of, e.g., ceramic or polymer material. The preparation of thecavity is made without an undercut. Undercuts can be blocked with somecement, e.g., glassionomercement.

The ceramic powder can be made by several well known methods.Traditional powder metallurgical techniques can be used, where thedifferent components are mixed and ground under dry or wet conditionswith water or an inorganic solvent (e.g., alcohols) as grinding liquid.The so-called SOL-GEL technique can also be used where different oxidematerials are deposited together from a water solution or areco-precipitated from metal alcoxides in e.g., water-free alcohol bycontrolled addition of water. A combination of different techniques canalso be used by using SOL-GEL technique to deposit a surface layer ofdesired metal oxide on a powder material. Lubricants or other organicbinders depending on the choice of forming method may be added to theceramic powder when needed at suitable times in the process as isconventionally known. Other preparation routes of the ceramic materialare also possible such as reaction sintering where a suitable metal isoxidized, nitrided, etc. For example, aluminum can be oxidized undercarefully controlled processing to alumina. These methods allowpreforming or reinforcement by fibers, e.g., in a felt infiltrated withliquid metal.

Many of the monolithic ceramics which are biocompatible may have abrittle performance if they are not sintered to nearly full density,more than 98% and preferably >99.5% of the theoretical density. However,these ceramics can be strengthened by a number of toughening mechanisms.Finely dispersed particles, platelets, whiskers or fibers raise thefracture toughness of the composite. Typical additives are the nitrides,carbides, borides or mixtures thereof of the transition metal of groupIV--VI or the elements Al or Si. Toughening may also be achieved byso-called transformation toughening, i.e., additions of unstabilizedZrO₂ or ZrO₂ stabilized with Y₂ O₃, MgO or CaO. The additions of theselatter oxides shall not exceed 25 wt %, but should be more than 2 wt %.The best performance is obtained with 3-12 wt % of the ZrO₂.

The powder with lubricants and/or other organic binders is coldisostatically compacted, uniaxially pressed, slip cast, pressure cast,injection moulded or compacted in another suitable way. The compactedbody has such dimensions that it comprises enough material for the copymilling of the outer shape of the core, which will fit into the preparedcavity E. During this copy milling, the sintering shrinkage must beconsidered. Thus, the copy milled surface D must be enlarged so that thecompacted body has such dimensions that after the shrinkage during thesubsequent sintering process to high density, it has the desired finalgeometrical external shape D which will fit into the prepared cavity Ewith great accuracy.

The ceramic body can also be presintered before the copy milling of thesurface D fitting to the prepared cavity E. All the other surfaces areprepared to near final shape before the final sintering. It is importantthat the ceramic material is sintered to closed porosity, which for anoxide material means at least 95% of theoretical density, but in orderto ensure good mechanical strength the material should preferably have adensity over 98%, while densities over 99.5% give the best strength.

The sintering can take place in a vacuum or under hydrogen atmosphere,under normal atmospheric pressure or under increased pressure inconnection with the overpressure sintering or hot isostatic compactionor alternatively by hot pressing. Highly pure Al₂ O₃ become translucentduring sintering to full density in vacuum or in hydrogen atmosphere,which is an advantage when natural teeth are to be imitated. Pure oxidematerial can be sintered in air, but some composites have to be sinteredin inert or controlled atmosphere. The core is given an external shapeso that the building-up of the veneer is facilitated. The external shapecan be such that it is roughly similar to natural teeth. After the finalsintering, the surfaces of the core may need some grinding, especiallythe external surfaces outside the prepared cavity. This grinding will bemade with the inlay or the onlay crown on a model of the prepared tooth.If dental porcelain is used as the veneer with a coefficient of thermalexpansion adapted to the material of the core, the porcelain will adherebetter. In the case of Al₂ O₃ as the core material there will be a"chemical bond" between Al₂ O₃ and porcelain. This means that theexternal surface of the core does not need any retention elements. Whenusing other veneer materials, e.g., plastic, mechanical retentionelements can be needed, e.g., grooves, pits or on the external surfacesintered retention elements or a silane treatment of the surface. Thecore can also be given such a shape that the ceramic inlay or onlaytooth crown does not need any veneer material. The surfaces of the inlayor the onlay tooth crown which is a part of the external surfaces of therepaired tooth must in this case before the cementing be ground andpolished to a surface fineness of 0.5-5 μm, preferably 0.5-1 μm.

The ceramic base material in the core comprises preferably one orseveral biocompatible oxides (including phosphates, silicates andsulfates), with the additives of carbides, silicides, nitrides orborides with or without binder metal (preferably iron-group metals) inaddition to conventional sintering aids. The base material can alsocomprise other biocompatible high performance ceramics such as nitrides,oxynitrides, cabides, etc. Examples of the two former materials are Si₃N₄, Si₂ N₂ O, sialon, AlN, AlON, etc. Examples of biocompatible oxides,which can form base matrix for the ceramic body, are Al₂ O₃, TiO₂, MgO,ZrO₂ (partly or totally stabilized with amounts of up to 25 weight % ofY₂ O₃, MgO or CaO).

Also, components such as SiC, TiN, TiC, TiB₂, Si₃ N₄, or otherbiocompatible carbides or nitrides of group IV, V or Vi can be presentas particles with a size of <25 μm, preferably <10 μm, and/or aswhiskers (hair shaped single crystals) with a length to diameterratio >5, preferably >10, and/or fibers (polycrystalline) with adiameter >10 μm and/or as single crystal platelets with an approximatediameter of 5-50 μm, preferably 50 μm, and a thickness of 1-10 μm,preferably 1-4 μm. The amount of whiskers, fibers and/or plateletsshould not exceed 60 volume preferably less than 40 volume %.

In a preferred embodiment, the ceramic material comprises >50%,preferably >85%, by weight of Al₂ O₃ with additives of conventionalsintering aids. In order to increase the strength <25 weight %,preferably 3-12 weight %, of ZrO₂, and/or 5-40 weight %, preferably10-30 weight %, of SiC whiskers can be added. In order to get a suitablecolor, colored components can be chosen. Additives, e.g., 0.1-10 weight%, preferably 0.5-5, weight %, of TiN and/or ZrN will give Al₂ O₃ basedinlays and onlay crowns a faint yellow shade.

The invention is additionally illustrated in connection with thefollowing Example which is to be considered as illustrative of thepresent invention. It should be understood, however that the inventionis not limited to the specific details of the Example.

EXAMPLE

A core to fit a prepared cavity in a molar according to FIG. 5 was madefrom a powder with the approximate composition 99.85 weight % of Al₂ O₃and 0.15 weight % MgO. Blocks (15 mm×15 mm×30 mm) of the powder wereuniaxially compacted and presintered at 1200° C. An impression was madeof the tooth with the prepared cavity with a puttymass of a siliconimpression material. This impression contained the negative reproductionD of the prepared cavity walls E. With a scalpel, the impressionmaterial outside the boundary I was carefully removed. From theimpression with use of the presintered blocks the surface D was copymilled and at the same time enlarged to such a size that it allowed ashrinkage of 16.5% during the sintering. From the boundary I theexternal surface of the core was milled and machined to a minimumthickness of the core of 1.2 mm in the direction of the axes through thetooth. The sintering was performed in air during 2 hours at 1600° C.After the sintering the core had a relative density of 99.5% and aminimum thickness of 1 mm in the direction of the axes through thetooth. An ordinary impression was made from the whole jaw and from thisimpression a model of the jaw was made in gypsum. The core fittedperfectly into the prepared cavity and commercially available dentalporcelain was fired on the surface. The first layer porcelain comprisedabout 50% Al₂ O₃ and was fired at 1150° C. during 10 minutes. During theheating the furnace was under vacuum, but when the final firingtemperature was reached the firing was performed under atmosphericpressure. The remainder of the crown was fired at 960° C. The dentalporcelain combined chemically with the alumina without any gap betweenthe porcelain and the densely sintered core. The onlay crown fittedperfectly on the model of the tooth and was ready to be cemented withconventional methods in the tooth cavity in the mouth of a patient.

The principles, preferred embodiments and modes of operation of thepresent invention have been described in the foregoing specification.The invention which is intended to be protected herein, however, is notto be constructed as limited to the particular forms disclosed, sincethere are to be regarded as illustrative rather than restrictive.Variations and changes may be made by those skilled in the art withoutdeparting from the spirit of the invention.

We claim:
 1. A method of making an artificial onlay tooth crown or inlayfor fit into a prepared tooth cavity comprising: forming a negativereproduction of said cavity; copy milling a core from said negativereproduction including a surface abutting said cavity and an externalsurface; and applying a veneer to at least part of the said externalsurface of said core.
 2. The method of claim 1 wherein said core whichis copy milled is a presintered body which is thereafter sintered.
 3. Amethod of making an artificial onlay tooth crown or inlay according toclaim 2 wherein the ceramic material of the core comprises at least oneof the oxides Al₂ O₃, TiO₂, MgO, ZrO₂ and ZrO₂ with up to 25 mole % ofat least one of Y₂ O₃, MgO and CaO.
 4. The method of claim 1 whereinsaid core comprises a biocompatible ceramic material with a relativedensity >95%.
 5. A method of making an artificial onlay tooth crown orinlay according to claim 4 wherein the ceramic material of the corecomprises at least one of the oxides Al₂ O₃, TiO₂, MgO, ZrO₂ or ZrO₂with up to 25 mole % of at least one of Y₂ O₃, MgO and CaO.
 6. A methodof filling a tooth cavity with an artificial onlay tooth crown or inlayforming an artificial onlay tooth crown or inlay comprising forming anegative reproduction of said cavity; copy milling a core from saidnegative reproduction including a surface abutting said cavity and anexternal surface; applying a veneer to at least part of the saidexternal surface of said core; and adhesively fitting said onlay toothcrown or inlay into said tooth cavity.